What is a Class?

In object-oriented programming, a class is a construct that is used as a blueprint (or template) to create objects of that class. This blueprint describes the state and behavior that the objects of the class all share. An object of a given class is called an instance of the class. The class that contains (and was used to create) that instance can be considered as the type of that object, e.g. an object instance of the "Fruit" class would be of the type "Fruit".

A class usually represents a noun, such as a person, place or (possibly quite abstract) thing - it is a model of a concept within a computer program. Fundamentally, it encapsulates the state and behavior of the concept it represents. It encapsulates state through data placeholders called attributes (or member variables or instance variables); it encapsulates behavior through reusable sections of code called methods.

More technically, a class is a cohesive package that consists of a particular kind of metadata. A class has both an interface and a structure. The interface describes how to interact with the class and its instances with methods, while the structure describes how the data is partitioned into attributes within an instance. A class may also have a representation (metaobject) at run time, which provides run time support for manipulating the class-related metadata. In object-oriented design, a class is the most specific type of an object in relation to a specific layer.

Programming languages that support classes subtly differ in their support for various class-related features. Most support various forms of class inheritance. Many languages also support features providing encapsulation, such as access specifiers.